The present invention relates to a solar heat collector, and more particularly to a selective absorption film formed on a heat collection plate of a solar heat collector.
Conventionally, metal members such as aluminum, copper, iron, and the like, which have good thermal conductivity properties, have been used as materials for heat collection fins of solar heat collectors, and metal members having a black film such as black nickel, black chromium, copper oxide, iron oxide, and the like have been practically employed.
In a solar heat collector that includes a heat collection tube and a heat collection fin sealed in a hollow cylinder of transparent glass, a heat collection fin having both a high absorption rate .alpha. of solar rays and a surface with a low radiation rate .epsilon. is generally necessary so as to collect the solar heat effectively. Particularly, a surface with a lower radiation rate .epsilon. and a selective absorption film with a higher absorption rate .alpha. are required for conventionally used vacuum hollow cylinder type solar heat collectors.
Further, it is desirable that the absorption rate .alpha. be high in air outside vacuum as in conventional flat plate type solar heat collectors, and that the radiation rate .epsilon. be low. However, it is difficult to mount a heat collection fin having a radiation rate .epsilon. of below 0.10 and a absorption rate .alpha. of above 0.90 on a vacuum hollow cylinder type solar heat collector, since a selective absorption film of black nickel, which is produced by electrolytically coloring an anodic oxidation film of an aluminum plate as conventionally employed, has an absorption rate .alpha. of 0.87 to 0.92 and a radiation rate .epsilon. of 0.12 to 0.20 under mass production. These characteristics are unsatisfactory for vacuum hollow cylinder type solar heat collectors.